1. Field of the Invention
The present invention relates to a pulse modulator, incorporated in a millimeter wave integrated circuit, a millimeter wave radar module of nonradiative dielectric waveguide type, or the like, for modulating a millimeter wave signal by ASK (Amplitude Shift Keying) or like modulation scheme, and also relates to a millimeter wave transmitter/receiver of nonradiative dielectric waveguide structure using the same.
2. Description of the Related Art
FIG. 8 shows the basic structure of a prior art nonradiative dielectric waveguide (hereinafter called the NRD guide) for transmitting therethrough a high frequency signal in the microwave or millimeter wave region. As shown, the structure comprises a dielectric waveguide 13 of rectangular or square cross section that is placed between parallel plate conductors 11 and 12 arranged in parallel to each other with a prescribed gap “a” provided therebetween. If this gap “a” satisfies the relation a ≦λ/2, the high frequency signal can be propagated through the dielectric waveguide 13 while preventing noise from entering the dielectric waveguide 13 from the outside and also preventing the radiation of the high frequency signal to the outside. Here, the wavelength λ of the high frequency signal is the wavelength in the air (free space) at the operating frequency.
FIG. 9A shows a perspective view of a pulse modulator to be incorporated in such an NRD guide, and FIG. 9B shows a plan view when the structure is viewed from above (refer to IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 46, NO. 6, JUNE 1998, pp. 806–810, “High-Speed ASK Transceiver Based on the NRD-Guide Technology at 60-GHz Band” (Futoshi Kuroki et al.)).
As shown, the pulse modulator comprises mode suppressors 20a, 20b, and 20c, two ferrite disks 21 for a circulator, and strip line conductors 22. The mode suppressors 20a, 20b, and 20c are each constructed from a dielectric waveguide made of Teflon (E. I. DU PONT DE MEMOURS AND COMPANY, Trademark; polytetrafluoroethylene), polystyrene, or like material, and shut off LSE (Longitudinal Section Electric) mode electromagnetic waves. The mode suppressors 20a, 20b, and 20c are arranged extending radially and spaced 120 degrees apart around the two ferrite disks 21 for the circulator. The strip line conductors 22, made of copper foil or the like, are formed inside the respective mode suppressors 20a, 20b, and 20c, and shut off LSE mode electromagnetic waves in which the direction of the electric field is perpendicular to the principal surfaces of the parallel plate conductors (in FIG. 9A, the direction from top to bottom). Each strip line conductor 22 is formed with a λ/4 choke pattern in order to eliminate TEM (Transverse ElectroMagnetic) mode.
At the opposite end of the mode suppressor 20b from the end connected to the ferrite disks, a dielectric waveguide 23a made of Teflon, polystyrene, or like material is disposed with a prescribed gap provided between it and that opposite end, and there is also disposed a dielectric sheet 24 made of alumina ceramics or like material having a different dielectric constant from that of the dielectric waveguide.
Behind the dielectric sheet 24 is disposed a dielectric wiring substrate 27 on which a strip line conductor 25 made of copper foil or the like is printed, with a Schottky barrier diode 26 mounted at an intermediate point along the strip line conductor 25 of choke-type bias supply line structure. A dielectric waveguide 23b made of Teflon, polystyrene, or like material is disposed behind the dielectric wiring substrate 27.
In the above structure, an electromagnetic wave propagated through the mode suppressor 20a is passed between the ferrite disks 21 where the wavefront is rotated in the clockwise direction so that the electromagnetic wave is directed into the mode suppressor 20b, not into the mode suppressor 20c. Then, the electromagnetic wave propagated through the mode suppressor 20b is absorbed at the Schottky barrier diode 26 on the dielectric wiring substrate 27 when a forward bias is applied to the Schottky barrier diode 26, but reflected when no bias or a reverse bias is applied to it.
The electromagnetic wave reflected by the Schottky barrier diode 26 is propagated back through the mode suppressor 20b and passed between the ferrite disks 21 where the wavefront is rotated in the clockwise direction so that the electromagnetic wave is directed into the mode suppressor 20c. In this way, by applying a bias voltage to the Schottky barrier diode 26, ASK modulation can be applied to the electromagnetic wave.
In the prior art pulse modulator for the NRD guide, however, since impedance matching is achieved for operation at the desired frequency by controlling the gap between the mode suppressor 20b and the dielectric waveguide 23a, the lengths of the dielectric waveguides 23a and 23b, and the thickness of the dielectric sheet 24, if there occurs a positional displacement among them or a manufacturing accuracy of them is low, the operating frequency may be displaced, degrading ASK modulation characteristics at the desired frequency. That is, the manufacturing accuracy and the positioning accuracy of these components have been difficult to manage, and this, coupled with poor fabrication reproducibility, has lead to low manufacturing efficiency; hence, the prior art has had the problem that it cannot ensure high reliability, nor is it suitable for mass production.
Furthermore, since the prior art pulse modulator for the NRD guide employs the structure in which the dielectric wiring substrate 27 with the Schottky barrier diode 26 mounted thereon is sandwiched between the dielectric sheet 24 and the dielectric waveguide 23b as shown in FIG. 9B, the prior art has had the problem that during fabrication the dielectric waveguide 23b may accidentally touch the Schottky barrier diode 26, damaging the Schottky barrier diode 26.
If such a pulse modulator is used for a millimeter wave transmitter/receiver, since ASK modulation is insufficient, there occurs the problem that millimeter wave isolation characteristics degrade and, when it is applied to a millimeter wave radar or the like, accurate detection becomes difficult.